Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
246 PM PDT Tuesday Jul 29 2014
Synopsis...an upper level ridge remains anchored over the Great
Basin and northern rockies. Upper level low pressure lingers off the
West Coast of Canada...as a result the Pacific northwest remains
under south to southwesterly flow aloft. The result is very weak
onshore flow...enough to affect the immediate coastal valleys while
the interior remains very warm. Weak disturbances lifting north from
California will bring a chance for thunderstorms at times to the
Cascades...especially Lane County. Little change expected in this
overall pattern the next several days.
Short term...today through Thursday night...thunderstorms have
developed along the Cascade crest this afternoon...with one
particularly large cell which was warned on near McKenzie Pass.
Radar hail size estimates maxed out at 1.75 inches...approaching
ping pong Ball size...though actual hail size was likely a bit lower
than that. 0-6 km bulk shear really not all that impressive...so
this storm appears to have fed a bit off of converging upslope winds
along the Cascade crest as it drifted northward. Perhaps one or two
more strong cells could develop similarly this afternoon...but we
are not anticipating this to be a widespread occurrence.
Another driver for the thunderstorm development this afternoon was a
weak disturbance and jet maximum pushing into the Oregon Cascades from
the southwest. This also may have helped push the above mentioned
cell to severe limits. As the jet maximum moves on and the sun GOES down
expect any thunderstorm activity to decrease rapidly this evening.
For most of the forecast area...the main story is and will continue
to be the very warm to hot weather inland. Temperatures are approaching or
have already surpassed 90 degrees at many inland locations as of 2
PM. While temperatures have cooled off nicely into the 50s in the outlying
valleys...urban areas and exposed hillsides have seen less relief
with temperatures only bottoming out in the 60s. There will be little change
in this over the next few days as an upper level ridge only gradually
weakens over the Great Basin and northern rockies. Coastal areas will
remain cooler as a weak onshore component to the flow continues.
Risk for thunder decreases temporarily Wednesday as most models
depict weak shortwave ridging transits the region. Still cannot
totally rule out thunder on the crest as lapse rates remain decent.
Moisture appears less abundant though...and again there should be
subsidence from the shortwave ridging. Chances increase again Thursday/Thursday
night as the remnant moisture of Post-tropical cyclone Hernan lift
north and possibly combine with one of the weak shortwaves presently
off the California coast beyond 130w. For now...most models suggest
this energy and moisture are targeting areas along the Cascade crest
and points east. However it is not out of the question areas further
west could be affected if the shortwave becomes strong enough to pull
in some southeast flow aloft. For now...just increased probability of precipitation a bit along the
crest of the Oregon Cascades Thursday/Thursday night. Weagle
Long term...Friday through Tuesday. Our area is on the west side of
an upper-level ridge over the western Continental U.S.. several shortwaves are
forecast to move up this ridge and bring chances for thunderstorms
along the Cascade crest. This ridge will remain stationary through
Tuesday. Uncertainty remains for timing and location of
thunderstorms...but looks like best chance will stay along the
Cascade crest.The stationary nature of this weather pattern will
keep high and low temperatures fairly persistent until this upper-
level ridge either breaks down or moves off to the east. -McCoy
Aviation...stratus and fog will continue to hug the coastline
with IFR conditions expected tonight into Wednesday morning. VFR
conditions for the inland airports tonight. A few isolated
thunderstorms possible along the Cascade crest this
evening...mainly south of Montana Jefferson. Some debris clouds
possible over the Cascades drifting north-northeast.
Kpdx and approaches...VFR prevailing tonight and Wednesday with
generally light north-northwest surface winds.
Marine...surface high pressure will remain over the NE Pacific
as a thermal trough holds over northern California. The thermal
trough will rise slightly north in the afternoon and evenings
resulting in a tighter pressure gradient over the Oregon waters.
Gusty winds can be expected at these times...occasionally reaching
or exceeding 25 knots. The gradient will be tightest south of
The seas will primarily be driven by the local winds. Steep seas 3
to 5 feet will dominate through the week. There are some
background longer period seas that may occasionally show up on
the buoys...but these are very small...1 foot or less...and less
significant to the mariner than the 3 to 5 feet seas at 6 to 8
seconds. The steep seas will be a combination of wind wave and
fresh swell which is nearly impossible to distinguish and have
therefore combined the seas in the forecast. The seas will be the
steepest when the winds are the greatest...during the afternoons
and evening...and especially south of Cascade Head and near shore.
Pz...Small Craft Advisory for winds until 11 PM PDT this evening for
coastal waters from Cape Shoalwater Washington to Cascade Head or
out 10 nm-coastal waters from Cascade Head or to Florence or
out 60 nm.
More weather information online at...http://weather.Gov/Portland
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This discussion is for northwest Oregon and southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.